This Invention relates to methods for the fabrication of gallium nitride compacts and other nitride compacts suitable for use as sputtering targets. Gallium nitride solid state devices are uniquely suited for the manufacture of "blue" light emitting diodes (LEDs) and of "blue light" lasers, including miniaturized lasers suitable for use in recording data, music or video signals onto compact discs. Microelectronic devices incorporating gallium nitride components can operate over broad temperature ranges, making them particularly suitable as electronic components in engines, furnaces and other equipment that operates at high temperatures. These developments have greatly increased the need for low cost, efficient methods of preparing gallium nitride source materials in a form suitable for use in established semiconductor fabrication techniques and apparatus.
Many different techniques and types of apparatus are used in the fabrication of electronic devices made of semiconductor materials. An important category of techniques, called sputter deposition, itself comprises several distinct methods including DC reactive sputtering and laser ablation. A common feature of these "sputtering" techniques is that a target, composed of a material to be deposited on a substrate, is bombarded with an energetic beam. This bombardment dislodges atoms or molecules of the target material from the surface of the target, and directs them to the surface of a nearby substrate, onto which the target material is deposited. The sputtering target is typically a wafer-shaped compact, produced using powder metallurgy compaction methods such as hot pressing, in which a powder mixture is subjected to a combination of high temperature and mechanical pressure to sinter the material into a bonded compact.
Gallium nitride targets usable in sputtering apparatus have heretofore not been available, however, for lack of any known method to manufacture a gallium nitride target usable for semiconductor applications. Efforts to prepare such gallium nitride targets using conventional powder metallurgy methods have been unsuccessful, in that portions of the gallium nitride starting material dissociates into gallium metal, nitrogen gas and gallium oxide, and the resulting targets generally demonstrate an unacceptably low density, with excessive porosity and contamination.
The failure of past attempts to prepare usable gallium nitride sputtering targets is attributed to problems arising from the low disassociation temperature of gallium nitride. Conventional powder metallurgy methods for the fabrication of compacts employ a combination of relatively high temperatures, in the order of 1,000.degree. C., and high pressures in the order of 20-50 Kpsi. However gallium nitride has a disassociation temperature of approximately 850.degree. C. at atmospheric pressure, and the disassociation of gallium nitride molecules brings about the venting of nitrogen gas and the reformation of gallium metal or (if oxygen is present) gallium oxide. As a result prior art attempts to make gallium nitride targets have resulted in compacts characterized by numerous pockets of gallium metal, and/or gallium oxide, and by voids resulting from the venting of disassociated nitrogen.
Experience has shown the necessity of producing sputtering targets of high density, whatever their composition, that are substantially free of material clusters and so-called black spots or holes. When targets having insufficient density are sputtered, overlarge particles in sizes from 0.70 .mu.m to 100 .mu.m may be ejected from the target and deposited on the thin film, causing a variety of defects that render the deposited material unsuitable for use in electronic applications. Accordingly the inability to fabricate gallium nitride sputtering targets that meet required density criteria has precluded the use of such targets in processes for fabricating electronic devices employing gallium nitride layers or components.
Extremely complex and expensive methods exist for depositing gallium nitride onto substrates, such as the molecular beam epitaxy method disclosed in U.S. Pat. No. 5,633,192 to Moustakas et al., and the chemical vapor deposition methods disclosed for example in U.S. Pat. No. 5,334,277 to Nakamura. However the prohibitive cost and the complexity of these methods render them effectively unusable for the fabrication of gallium nitride sputtering targets.
It is accordingly an object of the present invention to provide a method for the preparation of nitride compacts having the density and purity required for sputtering targets applications. More specifically it is an object of the invention to provide a method enabling the preparation of nitride targets that have improved physical characteristics, including a density in excess of 75% of theoretical maximum, a homogeneous nitride composition free of atomic metal or other contaminants, and improved mechanical characteristics that allow the target to be readily machined, ground, and polished.
It is a further object of the invention to provide a method for the preparation of nitride targets based on the conventional powder metallurgy technique of hot pressing, using as starting material a mixture of powders of a Ill-V nitride and of a nitrogen rich salt, preferably an ammonia salt.